In conventional plain paper electrostatic photography, an insulating photoconductor is charged with a corona source of ions, exposed, the charge image developed, the developed image transferred to plain paper, and finally, the toned image is fixed, generally by fusing. After the transfer operation, the residual image is erased from the surface of the photoconductor and the photoconductor is cleaned in preparation of a repetition of the process. Although employing plain paper, this process is complicated by the requirement for a number of different machine operations. In addition, the photoconductor suffers wear over a period of time, because the surface of the photoconductor is repeatedly rubbed by toner particles, cleansing brushes and paper surfaces.
A related process employs a photoconductively coated paper. The photoconductor, generally zinc oxide, is first charged, then exposed, and the image toned. The photoconductor is not reusable and thus the wear and tear restrictions are eliminated. In addition, the four-step machine operation is simplified. The disadvantage of this process is associated with the requirement for coating the paper with a photoconductor. Photoconductively coated papers are significantly more expensive than plain, uncoated paper. In addition, because of the heavy coating the papers are heavy and have a feel quite different from plain paper.
U.S. Pat. No. 3,797,926 teaches a solution of these problems by use of an improved corona modulating screen consisting of a plurality of strands with a photoconductive coating on the strands which is asymmetrical by being offset from a plane passing through the strands and perpendicular to the plane of the screen, when the screen is viewed in cross section.
There is an alternate teaching in this prior art U.S. Pat. No. 3,797,926, and that is directed to the concept of producing the toned image directly upon paper rather than first creating an electrostatic image which is later supplied with toner.
The teaching is that either a fully insulating dacron or nylon screen, or fully conducting phosphor bronze or stainless steel screen could be used as a mechanical carrier to position finely divided toner particles in the modulated corona stream. The suggestion is that the toner particles will pick up the charge normally sprayed onto the receiving sheet, and then the charged toner particles would be attracted to the carrier sheet rather than just the ions. This additional teaching ostensibly eliminates a separate toning step.